With such a forming process the exterior of the plate in the region of the central section to be formed is treated extremely carefully, because only this exterior, forming the visible side, comes into contact with the incompressible fluid work medium. This can be important for the visual appearance of a vehicle body. The shaped part keeps the desired precision since the shaping die contacts the metal plate directly on its non-visible rear side. In the case of a large-surface metal plate with weak curvature in its central section in order to obtain greater resistance to buckling and rigidity with minimum stress in the boundary section, it is also known to actively pre-arch the plate before the actual external high pressure punching, in particular against the subsequent drawing direction during the external high pressure punching (DE 197 17 953 A). As a result of stretching the material in the central section, generally accompanying it, higher degrees of elongation and a desired tensile strength can be achieved. While forming in the region of the central section of the metal plate is not problematic, it is comparatively difficult in the region of the outer edge because of the forming taking place in the region of the outer edge. Therefore, it is possible to obtain the large strain rates necessary in the region of the outer edge by pressing between profiling elements. Provided that the metal plate is made from a highly deep-draw capable material, the process can be carried out without difficulty. The trend with vehicle bodies, however, is moving towards high-strength steel (e.g., dual phase steel), in order to improve the crash behavior of vehicle bodies. Metal plates made from high-strength steel cannot be formed using the process described above without difficulty. Experience has shown that the metal plate tears in the boundary sections, where forming takes place by pressing between the profiling elements of the reservoir and the corresponding profiling elements of the shaping die.
With a further process for forming a large-surface metal plate into a shaped part of the type described in EP 1 147 833 A2 described above, the molding tool used has profiling dies arranged in its reservoir, which are arranged spread over the entire area and not just at the edge of the reservoir. Right from the start of forming, this process can work with low work fluid medium pressure, and thus even when the metal plate is contacted on the one side by the shaping die and on the other side only by the work fluid medium pressure. This work fluid medium pressure also remains constant if the forming process has progressed as far as the profiling elements of the reservoir and the shaped part is then pressed between these elements and the corresponding profiling elements of the shaping die. This means that during this phase the fluid work medium pressure is superimposed over the stamping pressure, which may lead to local overload on the material of the metal plate.